Application Notes

Due to growing restrictions on the use of lead in electronic products, efforts have been made to find appropriate substitutes. In the advanced IC packaging industry, the formerly ubiquitous, high-quality – but hazardous – eutectic SnPb solder bumps are now gradually being replaced by lead-free technology, such as SnAgCu alloy solder bumps. Because these new alloys require a certain composition in order to assure solderability and other mechanical properties, they must be measured precisely.

It is well known that the Ag and Cu content can exert profound impacts on the solderability and mechanical properties of Sn-based solder bumps. For instance, solder bumps with Ag content of more than 3% perform better in thermal fatigue testing and are more resistant to shear plastic deformation, while alloys with lower Ag content (around 1%) exhibit superior ductility and therefore better fatigue endurance under severe strain conditions. Furthermore, a mere 0.5% of Cu can decrease the dissolution behaviour of substrate Cu, thus increasing solderability.

That is why the IC packaging industry must accurately and precisely determine the composition of solder bumps, in order to fulfil the challenging combination of legal restrictions (being lead-free) and technical requirements.

The small size of the bumps (typically 80μm in diameter) prevents the use of most analytical methods. Others, such as atomic absorption (AA), are destructive and are therefore not suitable for testing each individual bump. However, X-ray fluorescence (XRF), has proven to be an ideal approach for monitoring the concentration of all three elements. Table 1 shows typical measurement results for a SnAgCu solder bump.

The X-ray beam of the FISCHERSCOPE® X-RAY XDV®-μ, equipped with poly-capillary optics and a silicon drift detector (SDD), can be focused down to measuring spot sizes as small as 20μm while still yielding very high count rates, guaranteeing outstanding repeatability and precision.

If accurately determining the composition of solder bumps – not only to verify lead-free technology – is important to you, the FISCHERSCOPE® X-RAY XDV-µ®, with its extremely small measuring spot, is the ideal instrument. For further information please contact your local FISCHER representative.

Controlling solder quality by measuring the remaining pure tin in coatings on PCBs

Because EU directives like EU2002/95/EC and EU2002/96/EC prohibit lead and other heavy metals, the solderable coating systems used on printed circuit boards must now be lead free. However, immersion tin carries the risk that, due to diffusion processes, the usable tin remaining in the plating can be insufficient to guarantee the success of solder processes and the quality of solder joints. Therefore the thickness of the pure tin in the coating must be checked before soldering.

Diffusion of copper into the tin starts immediately after deposition of the tin coating. Depending on temperature and time, intermetallic compounds can form which consume the tin in the plating until there is insufficient pure tin left to produce good solder joints. The loss of pure tin is further exacerbated by the heat of the solder process itself. For proper solder joints, a minimum thickness of 0.3 µm pure tin is required before the last solder procedure, meaning an initial layer of freshly deposited tin of at least 1-1.4 µm.

Fig.1: FISCHER COULOSCOPE® CMS

To ensure solderability, the thickness of the remaining pure tin in the plating must be measured precisely. The coulometric method (DIN EN ISO 2177) is the best choice for this task.

To demonstrate the diffusion problem with measurement results, PCBs with layers of ca. 0.5 and 1 μm immersion tin on top of various copper coating thicknesses were tempered and, after each heating procedure, measured with a FISCHER COULOSCOPE® CMS. The thickness of the copper coating exerted no influence on the thickness of the remaining tin.

Fig.2: The jump in de-plating potential between the pure tin coating and SnCu diffusion zone

The coulometric method makes obvious the drop in thickness of the pure tin coating. The measurement series with the 0.5 µm samples clearly shows that, even after only two hours of tempering, too little tin remains to guarantee proper solderability.

To check the solderability of coating systems on PCBs, the thickness of the remaining pure tin can be measured without being influenced by the SnCu-alloy using theFISCHERCOULOSCOPE® CMS. For more information please contact your local FISCHER representative.

Determining mechanical properties of thin CuSn6 foils

Bronze foils/strips are used for a huge variety of industrial applications, ranging from electrical contacts and membranes to spring elements and switches. The processing industry requires CuSn6 foils with more and more specific characteristics, e.g. significantly higher mechanical load-carrying capacity. To guarantee consistent quality, the mechanical characteristics of the foils must be determined.

The mechanical characteristics of thin metal foils can be determined using the instrumented indentation test (according to DIN ISO 14577). For this purpose the foils are affixed onto a smooth and stable surface. However, if the foil is applied unevenly or if an air bubble is trapped underneath it, the foil will bend while being measured, causing a false measurement of the indentation depth by adding an additional elastic but inconsistent percentage to the measurement result.

With the special foil clamping device from FISCHER, thin metal foils up to 200 µm in thickness can be easily braced over a cylinder, avoiding any critical fixation.

Measurements on three bronze strips (CuSn6) of different hardnesses were carried out with the aid of the foil clamping device. The thickness of the foils ranged from 75 μm to 170 μm. Table 1 shows the parameters measured for these strips.

HM(Martens hardness)N/mm2

HIT(indentation hardness)N/mm2

HV(Vickers hardnessderived from HIT)

EIT/(1-vs2)(elastic inden-tation module)GPa

ηIT(elastic deformation)%

strip 1

X.

1006

1179

111

97

9

s

19.9

27.6

2.6

10.7

1.0

V%

1.9

2.3

2.3

11.0

11.2

strip 2

X.

1116

1311

124

105

9

s

16.8

24.5

2.3

4.0

0.5

V%

1.5

1.9

1.9

3.8

5.4

strip 3

X.

1573

1916

181

108

12

s

71.8

88.5

8.4

8.0

0.8

V%

4.6

4.6

4.6

7.4

6.6

Tab.1: Exemplary measurement results for the mechanical properties of three CuSn6 strips. Displayed are the mean value, standard deviation and coefficient of variation for each parameter.

The mechanical properties of thin metal foils can be determined precisely and conclusively with the instrumented indentation test. For this purpose FISCHER provides the measuring system FISCHERSCOPE® HM2000 and a foil clamping device for optimal sample preparation. For further information please contact your local FISCHER representative.